Referring to FIG. 1, there is shown in simplified schematic diagram form a cellular telephone network 10 in which the present invention is intended for use. The three major components of the cellular telephone network 10 are a Mobile Switching Center (MSC) 16, a plurality of cell sites, or base stations, and a mobile unit, or station, 18. FIG. 1 shows cell sites A, B and C in the cellular telephone network 10. The MSC 16 is connected by wire to a Public Switched Telephone Network (PSTN) 14 shown in dotted line form in the figure. The PSTN 14 typically includes a large number of fixed telephones 12 for placing and receiving calls. The MSC 16 interfaces with the PSTN 14 and the cell sites via control links and voice trunks. The MSC 16 is also involved with switching calls between different cell sites in real time as the mobile unit 18 moves from the coverage area of one cell site to another. This process is known as a call "handoff" of the mobile unit 18 between adjacent cell sites.
A call handoff is performed in the following manner. Where the mobile unit 18 is served by cell site A, this cell site determines that the signal from the mobile unit is getting weak. Cell site A communicates the weakening of the received signal to the MSC 16 which then prompts neighboring cell sites B and C to measure the signal strength from the mobile unit and report their findings back to cell site A. Cell site A then selects the best set of neighbor cell sites that can accept the mobile unit 18 and requests a handoff from the MSC 16. The MSC 16 then informs the new cell site B that it is to accept a handoff and directs cell site B to tune to a selected radio channel being used by the mobile unit 18. The MSC 16 directs cell site A to direct the mobile unit 18 to tune to the new voice radio channel. The MSC 16 then switches the call from cell site A to cell site B, with the mobile unit 18 re-tuning to the new channel associated with cell site B.
The procedure described above is followed when a second cell site is available for handing off the mobile unit when the strength of the communications signal is too low to continue the call. In some cases, there may not be another cell site available for handing the mobile unit off to and the call is lost when the signal level is reduced to the point where the serving cell site can no longer maintain communications contact with the mobile unit. The present invention addresses this problem by alerting the mobile unit to a dangerously low signal level, allowing the user to take precautionary steps to avoid loss of the call.